Fix windows pacman repo build.

[acolwell]

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• [x] I've followed the contributing guidelines to the best of my understanding
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PR Description

What type of PR is this? (Check one of the boxes below)

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
  • [ ] I have updated the documentation accordingly

What does this pull request do?

Msys2 updated gcc in the last few days which has caused the package repo build to fail. Here are some minor fixes to make the build work again.

• Add fmt to openimageio's dependency list since the version included with the openimageio source now appears to cause an internal compiler error.

• Added an include for math.h to in a libsox test file to make the compiler stop generating an error about fabs() not being defined.

• Fix type mismatch and missing declaration errors in ImageMagick.

• Updated mingw-w64-dump_syms gitcommit to point to a version that fixes a missing header build error.

Have you tested your changes (if applicable)? If so, how?

All of the build actions have successfully run with these changes and pass their tests (build repo action results, build installer action results, CI tests action results)

I also downloaded the installer that was built and did some ad-hoc testing with a few projects. Everything seems to work as it did before.

Futher details of this pull request

This is intentionally a minimal set of changes to get things building again on Windows. Several of these fixes are present in more recent upstream versions. I'll likely update a few of these dependencies in a follow-up when I have some time so we can take advantage of upstream fixes.